Interpretive Summary: Soybean rust, a foliar disease which can cause seed yield losses of up to 80%, was first identified in the United States in November, 2004. Soybean cultivars with resistance to rust would be valuable in an integrated disease management program, since control of the disease currently relies solely on fungicide applications, but geographical and temporal variation in the virulence of the rust fungus has impeded breeding efforts. Between 2009 and 2012, plant introductions (PIs) from the USDA Soybean Germplasm Collection were screened for resistance to soybean rust at multiple locations in the southern USA as a collaborative project betweeen the USDA-ARS and five universities. The assays included some PIs with known rust resistance (Rpp) genes and others with unknown resistance genes which had shown at least moderate resistance in previouse evaluations. At least 78 PIs were resistant in Attapulgus, GA in 2012, and most of these had also been resistant at several locations in 2009. A large portion of these PIs were less resistant in Bossier City, LA in 2009 and in Quincy, FL in 2011 and especially 2012. Although several accessions were at least moderately resistant in all locations, including those with the Rpp1 and Rpp6 genes, this research shows that the pathogen causing soybean rust in the U.S. can vary significantly over time and locations. Identification and confirmation of germplasm accessions with consistent resistance to U.S. populations of the rust fungus will be useful to soybean breeders in making decisions about which crosses to make and which breeding populations to advance and evaluate in their efforts to develop rust-resistant cultivars for American soybean growers.

Technical Abstract:
Soybean [Glycine max (L.) Merr.] germplasm accessions from the USDA Soybean Germplasm Collection were screened for resistance to soybean rust (Phakopsora pachyrhizi) at up to five locations in the southeastern United States in 2009, 2011 and 2012. In 2009, plant introductions (PIs) from maturity groups III through IX were evaluated for relative disease severity and intensity of sporulation from uredinia compared to 12 susceptible cultivars from the same range of maturity groups. Resistance evaluations were based primarily on disease severity and intensity of sporulation from rust pustules. To assess resistance at several nurseries, a rust index (RI) score was calculated from the severity and sporulation ratings. Many of the PIs were moderately to highly resistant at the 2009 locations between Alabama and South Carolina, but the P. pachyrhizi population in Bossier City, LA, was more aggressive on most of those accessions. The 2011 rating data from Quincy, FL indicated an increase in the virulence of the pathogen there since 2009, and this trend was observed again in 2012. In contrast, many of the same PIs developed substantially less soybean rust in Attapulgus, GA in 2012. Despite the comparatively greater disease that many accessions had in Louisiana in 2009 and in Quincy in 2011 and 2012, at least 78 PIs were resistant in Georgia in 2012, and 20 of those were at least moderately resistant in both Florida and Georgia that year. Overall, PIs with either the Rpp1 or Rpp6 gene were the most resistant of the accessions with named resistance genes, and were among the most resistant accessions overall. No accessions were immune to rust, but PI 416826A, PI 417125, PI 567034 and PI 567104B consistently had effective levels of resistance in different locations and years. Information about the most resistant PIs and their reactions to soybean rust across years and locations will be useful for the development of rust-resistant soybean cultivars in the USA.